Sorry, netinsts are special - they only contain the installer and the
base system and a *very* limited set of extra packages that are
hand-configured.
The base system will end up overlapping a little with Priority:
standard (the 43 you've seen). That's all. You still get a basic
useful system with a netinst only, but it won't have all of "Standard"
included.

So, IIUC, the error can still happen with the netinst image if :
- the installer automatically adds the security archive
- the user does not select a mirror
- the user selects standard system utilities in tasksel

- and a package with standard priority is available in the security
archive, but has dependencies which are available only in the main archive.